The present invention relates to drop monitoring devices and in particular to such devices as used for counting drops of medicine or other infusion liquid in an intravenous delivery system, and operative to develop a signal upon the occurrence of a drop falling in a drip chamber in such a system.
Drop counting fluid delivery systems are known, and are in common use for delivery of all manner of infusions in a hospital or clinical setting. Such systems may be characterized in having a fluid reservoir, a drop forming chamber, and a fluid delivery tube extending, via a catheter, from the drop forming chamber into the vascular system of a patient. A drop sensor surrounds the drip chamber and senses the occurrence of a falling drop.
While several means of drop sensing are known, the prevalent mechanism is one in which a beam of light is directed across the drip chamber to a light detector, so that the beam is occluded each time a drop falls. Such systems, while of general practicality, have several inherent drawbacks. One is that because medical personnel prefer to visually confirm the operation of the system, the drip chambers are generally transparent, leading to a high level of ambient light. In such conditions, transient bursts of illumination, such as sunlight reflected from moving metallic objects, may operate to saturate the detector and blind it to an actually occurring drop, or may operate to trigger a false drop count. Solutions to this problem in the prior art involve elaborate signal averaging techniques, illumination stabilizing circuitry, and shielding of the portion of the drip chamber near to the detector/light elements. A further problem occurs in such systems because the trajectory of a falling drop follows a true vertical, whereas the drip chamber itself, being attached to an infusion tube, may be pulled at an angle off the vertical. The resulting effect is misalignment between the optimal path for drip detection and the actual drop path, leading to ambiguous or unreliable detection signals. Furthermore, because most infusion liquids are essentially transparent, the actual amount of light absorbed or scattered by a drop of such liquids may be a small percentage of the nominal light beam amplitude, so that signal discrimination is difficult, even in the absence of the other effects noted above.
The detection of drops can be frustrated by the accumulation of droplets on the wall of the drip chamber.
In such cases, a user flicks the chamber with his finger, causing a vibration to clear the accumulation from the wall.
One method known in the art for facilitating the counting of transparent liquid drops at low levels of general illumination is shown in U.S. Pat. No. 3,217,709 issued for an invention of J. H. Schneider et al. That patent shows a curved reflective clip which fits around a drip chamber and having a focus at the point of drop formation. The reflector is open on one side, so that when illuminated by the essentially parallel rays of a flashlight beam, all the incoming light is focussed upon the forming drop, causing it to appear bright. When the drop falls a sudden darkness is apparent. The device of that patent is configured for direct visual observation with both a light source and an observer far from the chamber, and is not adapted to an automated drip counting system. Also known in the art is the expedient of using a single mirror to reflect light across a drop path from a light source to a light detector in a photoelectrically operated drop counting apparatus. Such a configuration is shown in U.S. Pat. No. 3,563,090 issued for an invention of B. V. Deltour, at FIG. 4 thereof. The mirror in that device is apparently used simply to gain flexibility in the location of the various components (source, detector) within the drop detector housing; it does not show awareness of the signal enhancement possibilities using such a mirror.